docs: add input packing example (Consensys#1311)

examples/inputpacking/doc.go

@@ -0,0 +1,19 @@
+// Package inputpacking illustrates input packing for reducing public input.
+
+// Usually in a SNARK circuit there are public and private inputs. The public
+// inputs are known to the prover and verifier, while the private inputs are
+// known only to the prover. To verify the proof, the verifier needs to provide
+// the public inputs as an input to the verification algorithm.
+//
+// However, there are several drawbacks to this approach:
+//  1. The public inputs may not be of a convenient format -- this happens for example when using the non-native arithmetic where we work on limbs.
+//  2. The verifier work depends on the number of public inputs -- this is a problem in case of a recursive SNARK verifier, making the recursion more expensive.
+//  3. The public input needs to be provided as a calldata to the Solidity verifier, which is expensive.
+//
+// An alternative approach however is to provide only a hash of the public
+// inputs to the verifier. This way, if the verifier computes the hash of the
+// inputs on its own, it can be sure that the inputs are correct and we can
+// mitigate the issues.
+//
+// This examples how to use this approach for both native and non-native inputs. We use MiMC hash function.
+package inputpacking

examples/inputpacking/inputpacking_test.go

@@ -0,0 +1,199 @@
+package inputpacking
+
+import (
+	"crypto/rand"
+	"fmt"
+	"math/big"
+
+	fp_bn254 "github.com/consensys/gnark-crypto/ecc/bn254/fp"
+	fr_bn254 "github.com/consensys/gnark-crypto/ecc/bn254/fr"
+	cmimc "github.com/consensys/gnark-crypto/ecc/bn254/fr/mimc"
+
+	"github.com/consensys/gnark-crypto/ecc"
+	"github.com/consensys/gnark/backend/groth16"
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/frontend/cs/r1cs"
+	"github.com/consensys/gnark/std/hash/mimc"
+	"github.com/consensys/gnark/std/math/emulated"
+	"github.com/consensys/gnark/std/math/emulated/emparams"
+)
+
+func inCircuitComputation(api frontend.API, input1, input2 frontend.Variable, expected frontend.Variable) {
+	res := api.Mul(input1, input2)
+	api.AssertIsEqual(res, expected)
+}
+
+func inCircuitComputationEmulated(api frontend.API, input1, input2 emulated.Element[emulated.BN254Fp], expected emulated.Element[emulated.BN254Fp]) error {
+	f, err := emulated.NewField[emulated.BN254Fp](api)
+	if err != nil {
+		return err
+	}
+	res := f.Mul(&input1, &input2)
+	f.AssertIsEqual(res, &expected)
+	return nil
+}
+
+// UnpackedCircuit represents a circuit where all public inputs are given as is
+type UnpackedCircuit struct {
+	Input1, Input2                 frontend.Variable                  `gnark:",public"`
+	EmulatedInput1, EmulatedInput2 emulated.Element[emulated.BN254Fp] `gnark:",public"`
+	Output                         frontend.Variable                  `gnark:",private"`
+	EmulatedOutput                 emulated.Element[emulated.BN254Fp] `gnark:",private"`
+}
+
+func (circuit *UnpackedCircuit) Define(api frontend.API) error {
+	inCircuitComputation(api, circuit.Input1, circuit.Input2, circuit.Output)
+	return inCircuitComputationEmulated(api, circuit.EmulatedInput1, circuit.EmulatedInput2, circuit.EmulatedOutput)
+}
+
+// PackedCircuit represents a circuit where all public inputs are given as private instead and we provide a hash of them as the only public input.
+type PackedCircuit struct {
+	PublicHash frontend.Variable
+
+	Input1, Input2                 frontend.Variable                  `gnark:",private"`
+	EmulatedInput1, EmulatedInput2 emulated.Element[emulated.BN254Fp] `gnark:",private"`
+	Output                         frontend.Variable                  `gnark:",private"`
+	EmulatedOutput                 emulated.Element[emulated.BN254Fp] `gnark:",private"`
+}
+
+func (circuit *PackedCircuit) Define(api frontend.API) error {
+	h, err := mimc.NewMiMC(api)
+	if err != nil {
+		return err
+	}
+	h.Write(circuit.Input1)
+	h.Write(circuit.Input2)
+	h.Write(circuit.EmulatedInput1.Limbs...)
+	h.Write(circuit.EmulatedInput2.Limbs...)
+	dgst := h.Sum()
+	api.AssertIsEqual(dgst, circuit.PublicHash)
+
+	inCircuitComputation(api, circuit.Input1, circuit.Input2, circuit.Output)
+	return inCircuitComputationEmulated(api, circuit.EmulatedInput1, circuit.EmulatedInput2, circuit.EmulatedOutput)
+}
+
+func Example() {
+	modulusNative := ecc.BN254.ScalarField()
+	modulusEmulated := ecc.BN254.BaseField()
+
+	// declare inputs
+	input1, err := rand.Int(rand.Reader, modulusNative)
+	if err != nil {
+		panic(err)
+	}
+	input2, err := rand.Int(rand.Reader, modulusNative)
+	if err != nil {
+		panic(err)
+	}
+	emulatedInput1, err := rand.Int(rand.Reader, modulusEmulated)
+	if err != nil {
+		panic(err)
+	}
+	emulatedInput2, err := rand.Int(rand.Reader, modulusEmulated)
+	if err != nil {
+		panic(err)
+	}
+	output := new(big.Int).Mul(input1, input2)
+	output.Mod(output, modulusNative)
+	emulatedOutput := new(big.Int).Mul(emulatedInput1, emulatedInput2)
+	emulatedOutput.Mod(emulatedOutput, modulusEmulated)
+
+	// first we run the circuit where public inputs are not packed
+	assignment := &UnpackedCircuit{
+		Input1:         input1,
+		Input2:         input2,
+		EmulatedInput1: emulated.ValueOf[emparams.BN254Fp](emulatedInput1),
+		EmulatedInput2: emulated.ValueOf[emparams.BN254Fp](emulatedInput2),
+		Output:         output,
+		EmulatedOutput: emulated.ValueOf[emparams.BN254Fp](emulatedOutput),
+	}
+	privWit, err := frontend.NewWitness(assignment, ecc.BN254.ScalarField())
+	if err != nil {
+		panic(err)
+	}
+	publicWit, err := frontend.NewWitness(assignment, ecc.BN254.ScalarField(), frontend.PublicOnly())
+	if err != nil {
+		panic(err)
+	}
+
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &UnpackedCircuit{})
+	if err != nil {
+		panic(err)
+	}
+	pk, vk, err := groth16.Setup(ccs)
+	if err != nil {
+		panic(err)
+	}
+	proof, err := groth16.Prove(ccs, pk, privWit)
+	if err != nil {
+		panic(err)
+	}
+	err = groth16.Verify(proof, vk, publicWit)
+	if err != nil {
+		panic(err)
+	}
+
+	// print the number of public inputs when we provide all public inputs. Note that we also count the commitment here.
+	fmt.Println("unpacked public variables:", ccs.GetNbPublicVariables())
+
+	// then we run the circuit where public inputs are packed
+	var buf [fr_bn254.Bytes]byte
+	var buf2 [fp_bn254.Bytes]byte
+	h := cmimc.NewMiMC()
+	input1.FillBytes(buf[:])
+	h.Write(buf[:])
+	input2.FillBytes(buf[:])
+	h.Write(buf[:])
+	emulatedInput1.FillBytes(buf2[:])
+	h.Write(buf2[24:32])
+	h.Write(buf2[16:24])
+	h.Write(buf2[8:16])
+	h.Write(buf2[0:8])
+	emulatedInput2.FillBytes(buf2[:])
+	h.Write(buf2[24:32])
+	h.Write(buf2[16:24])
+	h.Write(buf2[8:16])
+	h.Write(buf2[0:8])
+
+	dgst := h.Sum(nil)
+	phash := new(big.Int).SetBytes(dgst)
+
+	assignment2 := &PackedCircuit{
+		PublicHash:     phash,
+		Input1:         input1,
+		Input2:         input2,
+		EmulatedInput1: emulated.ValueOf[emparams.BN254Fp](emulatedInput1),
+		EmulatedInput2: emulated.ValueOf[emparams.BN254Fp](emulatedInput2),
+		Output:         output,
+		EmulatedOutput: emulated.ValueOf[emparams.BN254Fp](emulatedOutput),
+	}
+	privWit2, err := frontend.NewWitness(assignment2, ecc.BN254.ScalarField())
+	if err != nil {
+		panic(err)
+	}
+	publicWit2, err := frontend.NewWitness(assignment2, ecc.BN254.ScalarField(), frontend.PublicOnly())
+	if err != nil {
+		panic(err)
+	}
+
+	ccs2, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &PackedCircuit{})
+	if err != nil {
+		panic(err)
+	}
+	pk2, vk2, err := groth16.Setup(ccs2)
+	if err != nil {
+		panic(err)
+	}
+	proof2, err := groth16.Prove(ccs2, pk2, privWit2)
+	if err != nil {
+		panic(err)
+	}
+	err = groth16.Verify(proof2, vk2, publicWit2)
+	if err != nil {
+		panic(err)
+	}
+	// print the number of public inputs when we provide only the hash. Note that we also count the commitment here.
+	fmt.Println("packed public variables:", ccs2.GetNbPublicVariables())
+	// output: unpacked public variables: 11
+	// packed public variables: 1
+}